1. Field of the Invention
This invention relates to a secondary battery of a proton conductive polymer, particularly the secondary battery wherein capacity characteristic and cycle characteristics are improved.
2. Description of the Related Art
A secondary battery of a proton conductive polymer consists of a construction, wherein a positive electrode is formed on a positive electrode collector and a negative electrode is formed on a negative electrode collector and these electrodes are piled up via a separator, and aqueous or non-aqueous solution containing a proton source as an electrolyte is filled therein.
A method of forming an electrode consists of preparing a slurry by adding an adhesive to the powder of a raw material polymer which is doped or not doped and a conductive adjuvant on a collector such as a conductive rubber sheet, injecting the slurry into a desired mold and forming an electrode having a desired electrode density and membrane thickness by a hot-press.
A battery is constructed by arranging the positive electrode and the negative electrode, thus formed, facing to each other via a separator.
An aqueous solution of sulfuric acid is particularly utilized as an electrolyte wherein the sulfuric acid supplies a proton to the system by dissociating in two steps.
H2SO4xe2x86x92H++HSO4xe2x88x92xe2x80x83xe2x80x83(1)
HSO4xe2x88x92xe2x86x92H++SO42xe2x88x92xe2x80x83xe2x80x83(2)
The dissociation constant (pK2) of the second step (Formula (2)) is 1.9 which is comparatively small and the dominant anion in the aqueous solution is HSO4xe2x88x92 of the first step (Formula (1))
FIG. 4 illustrates a graph showing a comparison of terminal voltages vs. capacities in the electrolyte between 40 wt % and 20 wt % sulfuric acid (Comparative Example 2 and 3 described later). The graph indicates that increasing the concentration of sulfuric acid improves the initial capacity; however, the cycle characteristics in the case of 40 wt % sulfuric acid are remarkably reduced as shown in FIG. 5.
In the secondary battery of a proton conductive polymer, HSO4xe2x88x92 is doped as a dopant in the polymer and a secondary battery having a high capacity can be formed by increasing the electric conductivity of the electrode material by raising the sulfuric acid concentration. However, since simply increasing the sulfuric acid concentration of the electrolyte reduces the cycle characteristics as described above, the sulfuric acid concentration of the battery of a proton conductive polymer is set up at about 20%.
The initial doping rate into the electrode at this sulfuric acid concentration (dopant concentration) was not sufficiently obtained and the capacity was not satisfactory enough because the concentration of the electrolyte was changed by an electrochemical doping accompanied by charge/discharge. Furthermore, the cycle characteristic was hardly sufficient and the low temperature characteristic was also insufficient.
The purpose of this invention is to provide a secondary battery of a proton conductive polymer having both an excellent capacity and excellent cycle characteristics.
As a result of studies to solve the problems as described above, the inventors found that the cause of the deterioration of the cycle characteristics when increasing the concentration of sulfuric acid for the purpose of a sufficient doping was due to promoting the deterioration of the electrode material by the state of peroxidation which was simultaneously brought about by the high proton concentration in the system. Therefore, in order to increase the dopant anion concentration and restrict an increase in proton concentration of the system simultaneously, a material was added which is not a source of a proton but can supply an anion equivalent to the anion species present in the system. In this way it became possible to increase only the dopant (anion species) concentration without changing the concentration of proton, raise the capacity and improve the cycle characteristics.
This invention relates to a secondary battery of a proton conductive polymer, wherein a positive electrode and a negative electrode are arranged facing to each other via a separator in an electrolyte and only a proton in an indole trimer and a xcfx80 conjugated polymer or a proton of a hydroxyl group in a hydroxyl-containing polymer as an active material of electrode in the positive electrode and in the negative electrode participates in a charge/discharge, and a proton concentration is 5 to 40% and an anion concentration is 30 to 60% in the solution, respectively, and the anion concentration is at least higher than the proton concentration.
Particularly when the electrolyte is an aqueous solution of sulfuric acid, an addition of a hydrogensulfate is preferable.
According to this invention, in the battery of a proton conductive polymer, the activity of the active material of the electrode can be increased sufficiently by increasing only the concentration of the dopant anion without increasing the proton concentration in the system and it is possible to restrict the volume change of the electrode and the structural deterioration, and provide a battery having a longer life than conventional one by having the same dopant species of the additive as the chemical species of the electrolyte. In addition, the low temperature characteristic is improved by the effect of molar depression of freezing point since the concentration of chemical species in the system is raised without increasing sulfuric acid concentration by adding the additive.